Solids inlet probes are being used for quantitative analysis in three areas. (1) Measurement of calcium stable isotope ratio using a thermal ionization technique. Tracer studies of calcium metabolism in animals and neonates are being carried out. (2) Analysis of amino acids using isobutane chemical ionization. Tracer studies using 13C alanine isotope ratios in animal models are being undertaken. (3) Slow pryrolysis techniques are being developed for analysis of peptide hormones. Initial quantitative studies with this technique are being carried out using acetate derivatives of phospholipids.